Steering wheels made of cardboard would be easier to manufacture, yet they'd fail safety inspection.
If car makers are actually hooking the bluetooth chip up to anything except the sound system, that's the problem. Although, assuming a class 2 device, it only has a 10 meter range. If an attacker is that close and wants to shut down your car, they can use a nail strip.
The real problem here is hooking up WWAN to control software.
These connections could be physically 'one-way' but in practice most modules are simply wired in parallel on a CAN bus and something like the radio would be set up to send data to the car's CAN bus (for diagnostic purposes at least).
The stereo is way too tightly coupled with the rest of the vehicle.